This invention relates to an electrical circuit for protecting sensitive electrical equipment and maintaining accuracy of operation thereof while the equipment is exposed to variations in magnitude and waveform of the power delivered from an external source. More particularly, the present invention utilizes a plurality of variable reactances in a network interposed between the external power source and the equipment to maintain a sinusoidal waveform for the applied voltage and to essentially eliminate the effect of transient phenomena on the equipment.
Present day electronic equipment is designed for carrying out extremely sensitive measurement of externalities and performing complex functions with the gathered data. The capabilities and sensitivity of the equipment appears to be increasing exponentially, most notably in the medical field. The introduction of tomographic scanning equipment has been rapidly followed by its widespread distribution and the development of expanded capabilities therefor. For example, the number of magnetic resonance imaging machines routinely employed throughout the United States as diagnostic tools has quickly increased. The medical equipment field is but one example of the expanding use of sensitive electronic equipment to obtain and process data.
While increasing reliance is being placed on the data generated by these electronic systems, the accuracy of operation, mean time between failure and equipment lifetime are becoming increasingly determined by the quality of the power delivered to the equipment. In the past, protective circuitry has been utilized to prevent voltage spikes in the power delivery system from causing damage to equipment. Typically, a shunt conductive path with variable loading is coupled across the terminals of the local power delivery network to limit the magnitude of transient voltage disturbances delivered to the equipment. The shunt path serves as a clamping circuit to limit voltage spikes to a predetermined level. Thus, a measure of protection against immediate failure is obtained since the input voltage is prevented from rising above the predetermined level.
While the effect of severe transient occurrences is reduce& transient phenomena tend to cause a series of subsequent oscillations or ringing effects that can be passed on to the equipment unless filtered or suppressed. The use of low pass filters in combination with diode pairs to partially counter these effects is disclosed in U.S. Pat. Nos. 4,095,163 and 4,156,838 to H. R. Montague. One circuit utilizes a series combination of zener diodes connected in opposition for damping oscillations, while the second circuit uses a parallel pair of opposing diodes. Each circuit includes a fixed series inductor and a fixed shunt capacitor in an attempt to divert high frequency transients.
Transient phenomena are also known to promote the flow of current surges in the shunt branch of the above-noted prior art circuits. These surges can be damaging to the components of the protective circuits thereby rendering the protective circuit inoperative and exposing the equipment to damage from subsequent transients. One approach to improving the reliability of protective circuits has been to add a magnetic element to the shunt circuit which presents a high impedance to high frequency components of transients while maintaining a low impedance at the frequency of the power supply. A more detailed discussion of this type of circuit is found in U.S. Pat. No. 4,434,396 to H. R. Montague.
Prior approaches to suppressing the major effects of high voltage transients on the power supply bus have been directed to overvoltage protection to prevent damage to the equipment and to the partial filtering of high frequency noise pulses on the input bus to prevent application of substantial transient effects to the load. These circuits have not been concerned with maintaining a sinusoidal waveform for the power delivered to the load. As a result, the waveforms of the voltage and current appearing at the input terminals of sensitive electronic equipment are subject to a variety of distorting effects. Waveform distortion can give rise to operating errors in the form of incorrect data or processing errors.
The problem of operating error leading to false data readouts is compounded by the fact that the loads themselves vary significantly during operation. As a result, a protective circuit interposed between the external power supply and a sensitive electronic instrument is required to maintain a sinusoidal waveform while encountering transient occurrences presented from the circuit input or output terminals.
Accordingly, a primary objective of the present invention is the provision of a protective circuit for maintaining the sinusoidal waveform of voltage and current supplied to a load. In addition, the circuit is intended for use with varying loads thereby suppressing transient occurrences taking place at both input and output terminals of the circuit. The present invention is well-suited for use with sensitive medical diagnostic equipment which receives power from the local utility grid and is therefore subjected to transient occurrences and waveform distortion resulting from variable loading at other locations on the grid.